1. Field of the Invention
The present invention relates to an electric brake device of the type that rotational motion from an electric motor is reduced by a reduction gear and is converted by a rotation-linear motion converting mechanism into a linear motion to press braking members against a brake rotor.
2. Discussion of the Related Art
As vehicle brake devices, as described in Japanese unexamined, published patent application No. 2002-213505, there has been known an electric brake device wherein a rotational motion from an electric motor is converted by a ball-screw mechanism into a linear motion to press braking members against a brake rotor.
In the vehicle brake devices of this kind, the electric motor is rotated in a positive-going direction at the time of a braking operation to move a screw shaft of the ball-screw mechanism toward the brake rotor side, but is rotated in a reverse direction at the time of a braking release operation to move the screw shaft toward the side away from the brake rotor. In the brake device described in the Japanese application, a problem arises in that the responsibility in the braking operation is deteriorated when friction members on the braking members are worn away, because a moving distance through which each friction member is moved until the same is pressed against the brake rotor is made to be longer.
As one for solving the aforementioned problem, there has been known an electric brake device which is provided with a wear compensating mechanism for the wears of friction members, as described in Japanese unexamined, published patent application No. 2005-233224. In the device, a through hole is provided in a screw shaft of a ball-screw mechanism, a strut shaft with a male screw large in lead angle is made to pass through the through hole with the male screw being in screw engagement with a screw hole formed on a part of the through hole, and a spring is provided for urging the strut shaft to be pressed against a stopper provided on a base end side.
In the brake device described in the last mentioned Japanese patent, when the electric motor is rotated at the time of a braking operation, the screw shaft of the ball-screw mechanism is moved forward to press a braking member against a brake rotor, and the strut shaft is moved together with the screw shaft. However, when separated from the stopper, the strut shaft being urged by the spring toward the base end side is loosened at the screw of the large lead angle and is moved relative to the screw shaft toward the base end side as it is rotated. Then, the strut shaft is again brought into abutting contact with the stopper and struts and supports the screw shaft not to allow the same to retract rearward. As a consequence, as the wear of the braking member proceeds, the pressing position of the same is gradually changed to be shifted forward, and with the change of the pressing position, the position of the screw shaft strutted and supported by the strut shaft is also changed forward. This advantageously makes it possible to hold the pressing portion of the screw shaft always close to the pressing position of the braking member, so that the responsibility in braking performance can be secured to be high.
However, in the brake device described in the last mentioned Japanese patent, the wear compensating mechanism is configured to be built in the pressing portion which is pressed against the brake rotor, and this unavoidably increases the mass of the pressing portion axially movable by the electric motor, so that the braking responsibility becomes liable to suffer an adverse effect. In addition, the wear compensating mechanism is arranged at a position close to a friction member (pad) on the braking member, and this gives rise to a problem that the brake device is susceptible to the influences of vibration and heat from the friction member.